WEDNESDAY, Feb. 8 (HealthDay News) -- Screening and treating expectant moms for thyroid problems at the end of the first trimester doesn't improve children's IQs at age 3, a new study finds.

Thyroid hormones are crucial for the development of a baby's brain and nervous system, and until the middle of the second trimester, those hormones come from the mother. Since a malfunctioning thyroid in the mother has been associated with mental impairments in her child, researchers reasoned that if they screened for maternal thyroid disease in pregnancy and treated any problems found, they might be able to improve function in the offspring. Treatment includes supplementary thyroid hormone medications such as levothyroxine.

However, this large study found that the intervention did not seem to help.

"This trial failed to show any beneficial effect on cognitive [mental] function in 3-year-old children whose mothers were receiving levothyroxine for impaired thyroid function -- mostly mild -- diagnosed at about 3 months of pregnancy," said study author Dr. John Lazarus, a professor of clinical endocrinology at the Centre for Endocrine and Diabetes Sciences at the Cardiff School of Medicine in Wales.

Results of the study are published in the Feb. 9 issue of the New England Journal of Medicine.

The thyroid is a small gland located in the front of the neck. It makes two hormones that are responsible for many functions in the body, including metabolism, brain development, breathing, heart functions, nervous system functions, blood cell creation, temperature regulation, muscle strength, bone health, skin dryness, menstrual cycles, weight, and cholesterol levels, according to the U.S. National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK).

Hypothyroidism (underactive thyroid) affects about three out of every 1,000 pregnancies, according to the NIDDK.

The study included almost 22,000 women from 10 centers in the United Kingdom and one center in Italy. The average length of pregnancy when the women were screening for thyroid disease was 12 weeks.

At the time of testing, half of the women were randomly assigned to the screening group and the other half served as a control group. A total of 390 women in the screening group tested positive for underactive thyroid, while 404 women in the control group did. Only the women in the screening group were given levothyroxine.

Children of women in both groups were given IQ tests at age 3 by psychologists who were unaware which group their mothers had been in.

Average IQ scores were 99.2 in the screening group and 100 in the control group. IQs of less than 85 were found in about 12 percent of the screening group and about 14 percent of the control group, according to the study.

Lazarus said there are several possible explanations for the findings. The thyroid screening and treatment initiation may have occurred too late in the pregnancy. Or, it may be that IQ is not a sensitive enough test to pick up subtle differences in children's mental function.

"Timing is a significant issue. This study was done at a time when the fetus doesn't supply its own thyroid hormone," noted Dr. Gregory Brent, a professor of medicine and physiology at the University of California at Los Angeles. So, it may be that by the time these women were screened and treated, it was too late to make a difference.

Lazarus added, "We don't know the effect on cognitive function if screening was done earlier. Another trial would be required."

Brent, who wrote an accompanying editorial in the same issue of the journal, also pointed out that most of the women in this study had a mild form of hypothyroidism. The researchers might have seen a statistically significant difference if the moms had more serious hypothyroidism.

Brent said the current guidelines from the major endocrine societies don't recommend universal screening for all pregnant women, but do encourage the screening of women who have a high risk of hypothyroidism. Those with a higher risk include women who have other autoimmune diseases -- such as type 1 diabetes, rheumatoid arthritis and lupus -- and those with a family history of thyroid disease.

And, although treating thyroid disease may not have an effect on early childhood cognition, Lazarus noted that treating thyroid problems may be important for a healthy pregnancy.

More information

For more on thyroid disease and pregnancy, visit the U.S. National Institute of Diabetes and Digestive and Kidney Diseases.

SOURCES: John Lazarus, M.D., professor, clinical endocrinology, Centre for Endocrine and Diabetes Sciences, Cardiff School of Medicine, Wales, U.K.; Gregory Brent, M.D., professor, medicine and physiology, University of California, Los Angeles; Feb. 9, 2012, New England Journal of Medicine

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